fix: docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192442
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192443

Author: snyk-bot

docs/requirements.txt

@@ -9,3 +9,4 @@ tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability
 pillow>=10.3.0 # not directly required, pinned by Snyk to avoid a vulnerability
 fonttools>=4.43.0 # not directly required, pinned by Snyk to avoid a vulnerability
 zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=2.6.0 # not directly required, pinned by Snyk to avoid a vulnerability